Neither company's name is disclosed in the filings, but AdExchanger punts that the telco could be Singtel, the Singapore-based company that has already spent big on ad tech acquisitions including Amobee, Adconion, and Kontera.

Singtel, AdExchanger notes, already invested $10 million in TubeMogul back in 2013, through its venture arm. Business Insider also heard rumors from several ad tech executives back in September that Singtel was on the hunt for a demand-side platform — a service TubeMogul offers — although the deal never materialized. A Singtel spokesperson told Business Insider in September that: "Singtel's policy is not to comment on rumors and speculation."

Was the social media company Facebook, Twitter, or Snapchat?

As far as the "social media company" is concerned, the SEC filing states a representative of TubeMogul's bankers Morgan Stanley met with the social media company rep on September 23 to discuss a potential acquisition, thanks to a "strong strategic fit" between the two.

Brett Wilson, TubeMogul CEO, spoke with the social media rep on September 26. However, the social media representative said "given a variety of priorities, [the social media company] might not be able to move quickly."

But discussions seemed to be progressing and on October 10, representatives of the social media company said it would want to focus on "technology, product, and finance due diligence" in order to come to a firm decision.

The social media company's "serious" interest continued into November 1, when its representatives had another call with TubeMogul. However, the deal stalled when the next day, the social media company told Morgan Stanley it wouldn't be able to make a decision within the requested timeline, "given other matters they were focused on."

So who was the social media company who saw a good strategic fit, but was too busy to pursue the deal?

Or perhaps it was Snapchat? Recode reported in January that Snapchat had been in early talks to potentially acquire ad tech companies including "bidder-as-a-service" startup Beeswax and programmatic analytics firm Metamarkets. In October, Snapchat opened up its ads API (application programming interface) to third-party ad tech firms — its first step into programmatic ad buying.

The SEC filing also reveals a "media company" and "a financial sponsor who owned a relative portfolio company" were also interested in buying TubeMogul.